Symposium S.EN02—Caloric Materials for Sustainable Cooling Applications

Cooling devices based on caloric materials have emerged as promising candidates to become the next generation of coolers. In recent years, several Electrocaloric (EC) heat exchangers have been proposed [1-5], covering different kinds of mechanisms and working principles. However, little has been told about the numerical modelling of these devices and their potential impact on improving the experimental performance. In this work, the authors highlight and prove how finite elements method could exploit and optimize EC heat exchangers. The simulations presented consisted of 2D-representations of a lead scandium tantalate multilayers capacitors (PST MLC) 22mm x 10.4mm x 1mm parallel-plates based active regenerator being developed at LIST and are used to explore new prototype’s configurations. Interestingly, results showed that by simplifying the complexity of the design and modifying the length and thickness of the parallel plates, the temperature span of the device could increase already from 1 to 9 degrees. By guidance of this outcome, the proposed new prototype’s configurations were tested in the laboratory. The maximum experimental temperature difference measured was of 9.45 degrees, which is several factors larger than the value obtained before the investigation of the numerical modelling. Furthermore, new modifications will be tested in the following months. According to our predictions, we expect to improve our actual maximum value even further.

Electrocaloric (EC) materials exhibit adiabatic temperature change or isothermal entropy when triggered with electric field. Their dimensionless materials efficiency, called EC materials efficiency, is the ratio between the heat generated and the electrical work needed to trigger the EC effect. By direct measurements of heat using infra-red (IR) camera, Differential Scanning Calorimetry (DSC), and electrical work we report quantitative results of the efficiency of Lead Scandium Tantalum (PST).These measurements are done on two different PST samples: thick bulk ceramic and thick multi-layer capacitors (MLCs). In both cases, we observed an adiabatic temperature change delta T_ad of 2.4 K at room temperature. For this delta T_ad, we report an efficiency of 35 and 85 respectively for MLCs and bulk. We present a comparative study on energy efficiency of different caloric materials, worked out indirectly in previous articles [1-2]. We reveal a maximum efficiency of 101 for PST (EC). Our study gives insights into the efficiency of EC materials and shows that PST is an excellent candidate for future efficient EC cooling devices.

[1] X.Moya, E.Defay, V.Heine and N.Mathur, Too Cool to work, Nature Physics, 2015.
[2] E.Defay, et al., Advanced Materials, 2013, 25, 3337-3342.

Since the discovery of giant electrocaloric (EC) effects [1-2], much effort has been made to explore new EC materials and develop prototypes EC cooling systems with high energy efficiency. However, the development of EC cooling systems has been one step behind magnetocaloric (MC) and elastocaloric (eC) cooling systems because few EC materials show comparable temperature change with MC materials. Moreover, EC working bodies with a large enough active volume and high breakdown strength have not been available.
Given this background, we have focused on developing EC multilayer capacitors (MLCs) based on various ferroelectric materials. MLCs are considered to be the ideal working bodies because a large electric field can be applied to thin EC layers without arcing, the active volume can be increased by laminating many thin EC layers, and existing MLC technologies are sufficient for mass production. The numerous internal electrodes permit EC effects to be driven by much larger electric fields than bulk ceramics, provided that the MLCs are of high quality.
Here we report EC effects driven by large electric fields in MLCs based on BaTiO₃ (BTO), (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT), (Pb,Ba)ZrO₃ (PBZ), (Pb,La)(Zr,Ti)O₃, and Pb(Sc,Ta)O₃ (PST) [3]. In 19-layer MLCs with layer thicknesses of 36-40 μm, fields exceeding 15 V µm^-1 yield adiabatic temperature changes of 2.7 K for 0.9PMN-0.1PT at 380 K [4], 3.8 K for PBZ at 430 K, and 5.1 K for PST at 300 K [5]. For MLCs of PST, process optimization and reduced EC layer thickness (18 μm) increases the breakdown strength to over 45 V μm^-1, which exceeds the 29 V μm^-1 that could be applied to MLCs of PST with thicker EC layers (38 μm) [5]. Our high quality MLCs should aid the development of prototype EC cooling systems that will be competitive with respect to their MC and eC counterparts.

References
1. A. Mischenko et al., Science 311, 1270 (2006).
2. B. Neese et al., Science 321, 821 (2008).
3. S. Crossley et al., Phys. Rev. X 9, 041002 (2019).
4. T. Usui et al. Phys., D: Appl. Phys. 50, 424002 (2017).
5. B. Nair et al., Nature (2019) doi:10.1038/s41586-019-1634-0.

Nowadays energy-intensive and expensive processes are mainly used to desalinate contaminated water and supply cooling capacity. Developing more sustainable, robust and cost-effective technologies (e.g. powered by renewable energy or waste heat) for generating clean water and cooling effect is a crucial need. This necessity is particularly important and severe in rural countries due to the lack of infrastructure and investments.
The abundant solar thermal source, which is now mainly employed to provide domestic hot water, could have a significant impact in achieving such targets, thanks to a wider engineering exploitation of innovative materials and technologies. [1,2,3] In this respect, passive distillation and cooling technologies – where all processes occur without mechanical moving parts – can make an important contribution. Unfortunately, state-of-the-art passive devices operating under one sun (that is, below 1 kW m^-2) still require efficiency enhancement, durability increase, and cost reduction to scale from lab to fab. [4,5,6] The passive technologies for solar water desalination and cooling developed at the Multi-Scale Modeling Laboratory of Politecnico di Torino (www.polito.it/small) are presented.
First, a fully passive, multistage and low-cost solar thermal device able to produce freshwater from seawater is conceived, prototyped and tested under both laboratory and outdoor conditions. The device does not require any mechanical moving parts by exploiting the capillarity of hydrophilic materials. Under realistic conditions, a distillate flow rate of almost 3 L m⁻² h⁻¹ from seawater at less than one sun is demonstrated. Theoretical models, under the same conditions, also suggested that the concept has the potential to further doubling the observed distillate rate, namely up to 6 L m^-2 h^-1. [7] Long-term operations of the passive distiller are then improved by optimizing the structure of evaporators towards enhanced salt-rejecting performance. [8] Finally, the passive distiller is optimized to recovery the low-temperature heat, to co-generate electricity and distilled water at the same time. [9]
Second, a passive device exploiting similar phenomena as i) capillarity of hydrophilic materials and the ii) salinity difference between multiple evaporators and condensers is experimentally demonstrated to provide cooling capacity up to 100 W m^-2. [10] The implemented prototype works at ambient pressure, employs environmentally friendly water solutions, and consists of several identical stages where the cooling effect comes from a cascade of evaporation-condensation processes driven by vapor pressure gradient. The experimentally validated model predicts that cooling capacities above 200 W m^-2 could be possibly achieved with a proper design optimization of the current lab-scale prototype.

Fasano, M., Humplik, T., Bevilacqua, A., Tsapatsis, M., Chiavazzo, E., Wang, E. N. and Asinari, P., Nature Communications, Vol. 7, 12762, 2016.
Ghasemi, H., Ni, G., Marconnet, A.M., Loomis, J., Yerci, S., Miljkovic, N. and Chen, G., Nature Communications, Vol 5, p.4449, 2014.
Morciano, M., Fasano, M., Salomov, U., Ventola, L., Chiavazzo, E. and Asinari, P., Scientific Reports, Vol. 7, 11970, 2017.
F. Signorato, M. Morciano, L. Bergamasco, M. Fasano, P. Asinari. submitted
Boriskina, S.V., Zandavi, S.H., Song, B., Huang, Y. and Chen, G., Optics and photonics news, Vol. 28, No. 11, 26, 2017.
Boriskina, S.V., Raza, A., Zhang, T., Wang, P., Zhou, L. and Zhu, J., MRS Bulletin, Vol. 44, No. 1, 59, 2019.
Chiavazzo, E., Morciano, M., Viglino, F., Fasano, M. and Asinari, P., Nature Sustainability, Vol. 1, No. 12, 763, 2018.
Ni, G., Zandavi, S.H., Javid, S.M., Boriskina, S.V., Cooper, T.A. and Chen, G., Energy & Environmental Science, Vol. 11, No. 6, 1510, 2018.
M. Morciano, M. Fasano, L. Bergamasco, A. Albiero, M. Lo Curzio, P. Asinari, E. Chiavazzo. submitted
M. Alberghini, M. Morciano, M. Fasano, F. Bertiglia, V. Fernicola, P. Asinari, E. Chiavazzo. submitted

The emergence of devices that combine rubbery elasticity with electronic, optoelectronic, and mechanically transductive properties offers exciting new opportunities for applications, but brings significant materials challenges. This presentation will describe our latest results in the materials efforts in developing highly compliant conductors, semiconductors, and dielectric polymers, as well as in the demonstration of integrated functional devices. Specific examples include a touch responsive sensors, a bistable electroactive polymer for tactile display, dielectric elastomer actuators, and compliant electrode for neural detection and stimulation.

The electrocaloric (EC) effect characterizes the change in temperature induced by a change in electric field [1], with the electrocaloric coefficient being defined as α=∂T/∂Ε│_S, where T is the temperature, E is the electric field and S is the entropy. It is expected that the EC effect has the potential to be an efficient solid-state cooling devices for a broad range of applications [2,3]. Up to now, fewer investigations about EC effects have been performed in relaxor ferroelectrics [4-8]. Here we study EC effect in Ba(Zr_0.5Ti_0.5)O₃ (BZT) and PbMg_1/3Nb_2/3O₃ (PMN) relaxor ferroelectrics. We use the first-principles-based effective Hamiltonian approach combined with Monte Carlo that has been recently developed for BZT and PMN solid solutions [9,10] to investigate the EC effects.

In the lead-free BZT systems, we investigated the electrocaloric effects subject to electric fields of different magnitude and all oriented along the pseudo-cubic [001] direction. It is found that, for any temperature, α exhibits a non-monotonic behavior with field that consists of small values at low fields, followed by an increase up to a maximum before decreasing for larger fields. Below the Burns temperature, this maximum of α is demonstrated to be correlated to a very specific microscopic feature. Finally, equalities that are derived from a simple Landau model reproduce and further help to understand the anomalous behavior of α with field and temperature in BZT, for any temperature above T_m where the dielectric response can exhibit a peak. Our simulations also confirm that indirect and direct approaches yield similar results of the α EC coefficient for any temperature above the T_m temperature but differ from each other for temperature below T_m, because of the non-ergodicity adopted by BZT at these low temperatures.

We also employed the effective Hamiltonian of Ref. [10] to shed some light on electrocaloric effects in PMN. It is particularly striking that such Hamiltonian can qualitatively reproduce not only the peculiar electric field-versus-temperature phase diagram but also the optimization of the EC coefficient near the critical point in this rather complex system. The fact that the recently developed Landau-like model, predicting that the EC coefficient is simply related to the product of temperature and the field derivative of the square of the polarization [7], also describes well the EC behavior of PMN as a function of electric field and temperature is also promising for phenomenological modelization of complex inhomogeneous systems. We therefore hope that our studies [7,8] lead to a broader knowledge of EC effects in ferroelectrics.

This work is supported by the National Natural Science Foundation of China (Grants No. 11804138 and No. 11825403), Shandong Provincial Natural Science Foundation (Grant No. ZR2019QA008), China Postdoctoral Science Foundation (Grant No. 2018M641905), and Shanghai Super Postdoctoral Incentive Program. We also thank the DARPA Grant No. HR0011-15-2-0038 (MATRIX program) and ARO grant W911NF-16-1-0227.

References:
[1] J. F. Scott, Annu. Rev. Mater. Sci. 41, 229 (2011).
[2] Y. Bai, G.-P. Zheng, and S.-Q. Shi, J. Appl. Phys. 108, 104102 (2010).
[3] X. Moya, S. K.-Narayan, N. D. Mathur, Nat. Mater. 13, 439 (2014).
[4] S. G. Lu et al., Appl. Phys. Lett. 97, 202901 (2010).
[5] R. Pirc, Z. Kutnjak, R. Blinc, and Q. M. Zhang, J. Appl. Phys. 110, 074113 (2011).
[6] B. Rozic et al., J. Appl. Phys. 110, 064118 (2011).
[7] Z. Jiang, S. Prokhorenko, S. Prosandeev, Y. Nahas, D. Wang, J. Íñiguez, E. Defay, and L. Bellaiche, Phys. Rev. B 96, 014114 (2017).
[8] Z. Jiang, Y. Nahas, S. Prokhorenko, S. Prosandeev, D. Wang, J. Íñiguez, and L. Bellaiche, Phys. Rev. B 97, 104110 (2018).
[9] A. R. Akbarzadeh, S. Prosandeev, E. J. Walter, A. Al-Barakaty, and L. Bellaiche, Phys. Rev. Lett. 108, 257601 (2012).
[10] A. Al-Barakaty, S. Prosandeev, D. Wang, B. Dkhil, and L. Bellaiche, Phys. Rev. B 91, 214117 (2015).

We present barocaloric studies of ferroelectric polymers of poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE]. There copolymers are well known because of their giant electrocaloric response, but here we study pressure-driven thermal changes. PVDF-TrFE polymers undergo first-order phase transition with very large latent heats and volume changes, near room temperature. These three properties make them ideal materials for barocalorics.

Barocaloric materials are blossoming, with recent reports of thermal changes that compare to those observed in commercial refrigerant fluids [1-3]. The viability of future barocaloric chillers will rely on properties that go beyond the magnitude of the pressure-driven barocaloric effects. On the one hand, the challenge of effective transmission of hydrostatic pressure needs to be addressed in order to guarantee homogeneous stress distribution in the active material, and therefore optimal caloric performance. On the other hand, heat extraction needs to occur at rates that can match the practical cycling frequencies. Here, I will discuss key aspects of pressure transmission and heat transfer in barocaloric devices, and present a guide for barocaloric materials to match applications requirements.

[1] X. Moya, et al. "Use of barocaloric materials and barocaloric devices", patent WO2018/069506 (2018).
[2] Li, Bing, et al. "Colossal barocaloric effects in plastic crystals." Nature 567.7749 (2019): 506.
[3] P. Lloveras, et al. "Colossal barocaloric effects near room temperature in plastic crystals of neopentylglycol." Nature communications 10.1 (2019): 1803.

The massive use of vapor-compressor-based coolers poses a serious global environmental problem due to a relatively small efficiency and the use of hydrofluorocarbons with huge greenhouse potential [1]. These devices are based on controlling the exchange of latent heat involved in liquid-vapor transitions by means of hydrostatic pressure. Alternative technologies based on driving solid-state first-order phase transitions using magnetic, electric and uniaxial stress fields avoid the use of harmful fluids and promise higher efficiencies [2], but they are limited by a relatively small magnitude of the latent heat associated with such transitions compared to evaporation. Here we show that pressure-driven caloric (i.e. barocaloric) effects at phase transitions in some plastic crystals is comparable to that in hydrofluorocarbons due to the emergence of strong molecular orientational disorder [3,4]. We also discuss the reversible barocaloric response that is obtained upon cyclic application and removal of pressure as this is key for technological implementation. In particular, we characterize reversible effects in terms of magnitude, temperature spans, and minimum required pressure. We determine that, while some of the plastic crystals under analysis are unfeasible for applications due to irreversibility within a large pressure range, others demonstrate colossal barocaloric effects that are reversible at low pressures [3,4] and therefore show aptitudes that should promote the future development of environmentally-friendly solid-state barocaloric cooling devices.

[1] https://www.birmingham.ac.uk/Documents/college-eps/energy/Publications/2018-clean-cold-report.pdf
[2] X. Moya et al., Nat. Mater. 13, 439 (2014).
[3] P. Lloveras et al., Nat. Commun. 10, 1803 (2019).
[4] A. Aznar et al., submitted (2019).

Abstract body: Spin crossover occurs in compounds where the crystal field splitting of d-orbitals associated with a magnetic moment is of the order of k_BT. As a result, a change of state from low spin (LS) to high spin (HS) can occur at the so-called spin crossover temperature, T_SCO. Crucially for caloric applications, the change of state from LS to HS can be either continuous or first order, and it can occur at temperatures up to and beyond room temperature. This makes SCO transitions of interest for caloric cooling applications. Since SCO compounds are paramagnets or diamagnets, the largest caloric effects will be barocaloric rather than magnetocaloric.
In this presentation, we provide the first direct experimental demonstration of a giant, low pressure barocaloric effect (BCE) and outline future research directions for the development and optimization of the BCE associated with spin crossover.

In this talk, we will discuss our latest work on development of elastocaloric materials as well as cooling prototype systems. We have used additive manufacturing to engineer NiTi-based composites with extremely extremely small hysteresis in stress-strain curves. Additive manufacturing allows flexible design topology of metallic refrigerants which also act as heat exchangers. The small hysteresis results in substantially enhanced materials coefficient of performance as well as long fatigue life needed for applications. On the systems front, we have demonstrated active regeneration schemes for systems with cooling capacity up to 100s of watts using commercially available NiTi tubes. The largest temperature span we have achieved for a single stage compressive regenerators is 23 K. This work is carried out in collaboration with H. Hou, J. Cui, R. Ott, A. Stebner, D. Catalini, R. Radermacher, and Y. Hwang, N. Emaikwu, and S. Qian,

The refrigeration and air conditioning consumes about 17% of the overall electricity used worldwide, and will drastically increase in the next 30 years. The vast majority of these systems utilize vapour-compression technology, which is actually and unfortunately still associated with the existing refrigerants and their environmental impact. Since they contribute to the 7.8 % of global greenhouse gas emissions, large efforts are invested in finding appropriate future working substances. However, the options for new, environmentally friendly refrigerants are very limited. Another important step in decreasing the energy consumption is through the energy efficient refrigeration and air conditioning. The large scale vapour compression systems operate with their upper limit of about 60% of the second law efficiency. However they suffer by the low 20% efficiency especially in small appliances. The above issues represent the driver for the quest for energy efficient and environmentally friendly refrigeration and air-conditioning, which is pronounced more than ever. Among the available best alternatives to existing technologies is the caloric energy conversion. This is due to the potentially very high energy efficiency and use of environmentally friendly and safe solid-state working substances. Most of today’s caloric devices apply so-called active regeneration, which involves the oscillation of the working fluid through the matrix of the caloric material – the caloric regenerator. However, the unavoidable, irreversible viscous and heat-transfer losses apply limits to the caloric device’s performance as well as its size. The quest for better caloric-device performance has led to the development of thermal control elements, which control the heat flux on different size and time scales. Here we describe the working principles of these elements: thermal switches, thermal diodes and thermal regulators. This is followed by the critical review of the research activities and applications of thermal control elements in all types of caloric devices. It is shown that thermal control elements have the potential to substantially improve the power density of caloric devices. Finally, we present the target features and future research guidelines for these elements in the field of caloric technologies.

Cooling, manifested both as air conditioning and as refrigeration, is regarded as one of the most energy-intensive technologies in demand today. Magnetic refrigeration is an energy-efficient, sustainable, environmentally-friendly alternative to conventional vapor-compression cooling technology. There are a number of magnetic refrigerator device designs in existence today that are predicted to be highly energy efficient, on condition that the suitable working materials can be developed.¹ This materials challenge has proven to be daunting due to issues associated with intricate synthesis/post-processing protocols and complications related to shaping the mostly brittle magnetocaloric alloys into thin-walled, channeled regenerator structures to facilitate efficient heat transfer between the solid refrigerant and the heat exchange fluid in an active magnetic regenerator (AMR) cooling device. To this end, research centered on fabrication of functionally-graded AlFe₂B₂ magnetic regenerators via sophisticated additive manufacturing (AM) methods will be presented in this talk. This ferromagnetic intermetallic boride has attracted considerable attention due to its low cost, promising thermal properties that promote effective heat transfer (specific heat capacity, C_p=115 J/moleK; thermal conductivity, κ=5.6 W/mK) and moderate room temperature magnetocaloric response (adiabatic temperature change, ΔT_ad=1 K and magnetic entropy change,ΔS=2.6 J/kgK at μ₀H_app=2 T).^2,3

In this work, powders of single phase AlFe₂B₂ were synthesized using multiple conventional routes: (i) drop-casting, followed by heat treatment at 1040 °C for 72 hours and then grinding to particulate form in air, (ii) reaction sintering at 900 °C for 24 hours, followed by etching in dilute HCl for 30 min and a (ii) molten salt-assisted solid state reaction at 600 °C for 36 hours. Experimental data indicate that irrespective of processing method, powders of dimensions ranging from 5 to 40 μm demonstrate comparable structural and magnetic properties (orthorhombic Cmmm-type crystal structure with unit cell volume, V=92.6 ± 0.4 Å; Curie temperature, T_c=277 ± 6 K) as well as similar magnetocaloric response (ΔS=2.6 ± 0.3 J/kgK at μ₀H_app=2 T). These precursor powders were used to 3D-print cylindrical AlFe₂B₂ samples using the established direct energy deposition AM method, laser enabled net shaping (LENS) and using an innovative magnetic-field assisted extrusion method whose prime novelty lies in the 3D ink formulation.^4,5 Efforts to optimize the material and printing process parameters to maximize the magnetocaloric potential of AlFe₂B₂ will be discussed. Further, the feasibility of printing high-quality magnetocaloric device components with high surface area and complex porous architectures will be deliberated upon. Overall, this study provides strategies for realizing low-cost functionally graded magnetic regenerators, thus potentially eliminating one of the main barriers to commercialization of magnetic cooling technology.

References: ¹V. Franco et. al., Annu. Rev. Mater. Res., 42, pp. 305, 2012; ²L. H. Lewis et.al., J. Alloys Compd. 650, pp. 482, 2015; ³ R. Barua et al., J. Alloys Compd., 777, pp. 1030, 2019; ⁴ B. Lejeuene et. al., Manuscript in preparation; ⁵ Zhao, Barua and Hadimani, Patent application #62.982,002.

Conventional refrigeration based upon the vapor compression cycle is an inefficient process when compared to the efficiency of a Carnot engine operating between the same thermal reservoirs. A viable alternative is magnetic refrigeration, which utilizes adiabatic magnetization and demagnetization to create a cooling cycle analogous to that of the vapor compression cycle through the ‘magnetocaloric effect’. A viable compound for magnetic refrigeration at ambient temperature must present a large change in magnetic entropy, with minimum hysteresis and should be composed of earth abundant and non-toxic elements.

Recently a computational proxy was developed to identify new magnetocaloric materials whereby first principle structure relaxation is carried out with and without spin polarization to ascertain the degree of magnetic deformation.[1] By applying this method to ferromagnet in the PbFCl family we found magnetic deformation of around 2% which is similar to other magnetocaloric compounds. We therefore have focussed our experimental study on MnZnSb, which is reported to be an itinerant ferromagnet with a second order phase transition and Curie temperature about room temperature.[2] Detailed magnetization measurements were carried out and Arrott analysis yields a Curie temperature of 304K, with the system being described as a 3D itinerant ferromagnet from the critical exponents of the Arrott-Noakes equation. We find a reasonably large magnetic entropy change of 4.5 Jkg^-1K^-1 with a relative cooling power of 153 Jkg^-1; which is comparable to second order compounds with about room temperature transition temperatures. Heat capacity measurement shows an anomaly around the magnetic transition and together with the magnetometry can be used to calculate an adiabatic temperature change of 2 K under 2 T. Temperature dependant powder neutron diffraction was used to investigate the origin of the significant magnetic entropy change around the magnetic transition. By alloying this compound with transition metal such as Fe and Cr we found a linear relationship between the c/a ratio and the magnetic transition allowing for the tuning of the operating temperature of this material from 250 K to 320 K without being too detrimental to the magnetic entropy change.

[1] J. D. Bocarsly, E. E. Levin, C. A. C. Garcia, K. Schwennicke, S. D. Wilson and R. Seshadri, "A Simple Computational Proxy for Screening Magnetocaloric Compounds," Chemistry of Materials, vol. 29, pp. 1613-1622, 2 2017.
[2] S.-i. Mori, T. Kanomata, H. Yamauchi, S. Sakatsume and T. Kaneko, "Magnetic Properties of MnZnSb," Japanese Journal of Applied Physics, vol. 32, p. 273, 1 1993.

Foams offer extremely large surface area per unit mass, making them competitive material for heat exchangers and energy storage systems in automotive applications. Understanding the influence of foam characteristics, i.e., size, distribution and the concentration of pores, on their effective thermal conductivity is important when designing the foam-based devices. Herein, we present the effective thermal conductivity of 15μm-thick nickel (Ni) coated open-cell polyurethane (PU) foam (20 PPI, with ~174μm truss cross-section edge length) architectures measured by transient plane source (TPS) method. A calibration methodology for TPS method is developed to ensure an accuracy in measurements. The experiment results are compared with theoretical models including a finite element model and effective medium theory. For modeling, an X-Ray tomography is used to accurately analyze the foam structures. We investigate how the foam characteristics, including the Ni coating thickness, coating defects, and angular alignment of the foam truss, modulate the effective thermal conductivity. Lastly, we discuss how the model assumptions are related to the discrepancy between the model predictions and measurements for the polymer-metal foams.

Cooling devices based on caloric materials have emerged as promising candidates to become the next generation of coolers. In recent years, several Electrocaloric (EC) heat exchangers have been proposed [1-5], covering different kinds of mechanisms and working principles. However, little has been told about the numerical modelling of these devices and their potential impact on improving the experimental performance. In this work, the authors highlight and prove how finite elements method could exploit and optimize EC heat exchangers. The simulations presented consisted of 2D-representations of a lead scandium tantalate multilayers capacitors (PST MLC) 22mm x 10.4mm x 1mm parallel-plates based active regenerator being developed at LIST and are used to explore new prototype’s configurations. Interestingly, results showed that by simplifying the complexity of the design and modifying the length and thickness of the parallel plates, the temperature span of the device could increase already from 1 to 9 degrees. By guidance of this outcome, the proposed new prototype’s configurations were tested in the laboratory. The maximum experimental temperature difference measured was of 9.45 degrees, which is several factors larger than the value obtained before the investigation of the numerical modelling. Furthermore, new modifications will be tested in the following months. According to our predictions, we expect to improve our actual maximum value even further.

Electrocaloric (EC) materials exhibit adiabatic temperature change or isothermal entropy when triggered with electric field. Their dimensionless materials efficiency, called EC materials efficiency, is the ratio between the heat generated and the electrical work needed to trigger the EC effect. By direct measurements of heat using infra-red (IR) camera, Differential Scanning Calorimetry (DSC), and electrical work we report quantitative results of the efficiency of Lead Scandium Tantalum (PST).These measurements are done on two different PST samples: thick bulk ceramic and thick multi-layer capacitors (MLCs). In both cases, we observed an adiabatic temperature change delta T_ad of 2.4 K at room temperature. For this delta T_ad, we report an efficiency of 35 and 85 respectively for MLCs and bulk. We present a comparative study on energy efficiency of different caloric materials, worked out indirectly in previous articles [1-2]. We reveal a maximum efficiency of 101 for PST (EC). Our study gives insights into the efficiency of EC materials and shows that PST is an excellent candidate for future efficient EC cooling devices.

[1] X.Moya, E.Defay, V.Heine and N.Mathur, Too Cool to work, Nature Physics, 2015.
[2] E.Defay, et al., Advanced Materials, 2013, 25, 3337-3342.

Since the discovery of giant electrocaloric (EC) effects [1-2], much effort has been made to explore new EC materials and develop prototypes EC cooling systems with high energy efficiency. However, the development of EC cooling systems has been one step behind magnetocaloric (MC) and elastocaloric (eC) cooling systems because few EC materials show comparable temperature change with MC materials. Moreover, EC working bodies with a large enough active volume and high breakdown strength have not been available.
Given this background, we have focused on developing EC multilayer capacitors (MLCs) based on various ferroelectric materials. MLCs are considered to be the ideal working bodies because a large electric field can be applied to thin EC layers without arcing, the active volume can be increased by laminating many thin EC layers, and existing MLC technologies are sufficient for mass production. The numerous internal electrodes permit EC effects to be driven by much larger electric fields than bulk ceramics, provided that the MLCs are of high quality.
Here we report EC effects driven by large electric fields in MLCs based on BaTiO₃ (BTO), (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT), (Pb,Ba)ZrO₃ (PBZ), (Pb,La)(Zr,Ti)O₃, and Pb(Sc,Ta)O₃ (PST) [3]. In 19-layer MLCs with layer thicknesses of 36-40 μm, fields exceeding 15 V µm^-1 yield adiabatic temperature changes of 2.7 K for 0.9PMN-0.1PT at 380 K [4], 3.8 K for PBZ at 430 K, and 5.1 K for PST at 300 K [5]. For MLCs of PST, process optimization and reduced EC layer thickness (18 μm) increases the breakdown strength to over 45 V μm^-1, which exceeds the 29 V μm^-1 that could be applied to MLCs of PST with thicker EC layers (38 μm) [5]. Our high quality MLCs should aid the development of prototype EC cooling systems that will be competitive with respect to their MC and eC counterparts.

References
1. A. Mischenko et al., Science 311, 1270 (2006).
2. B. Neese et al., Science 321, 821 (2008).
3. S. Crossley et al., Phys. Rev. X 9, 041002 (2019).
4. T. Usui et al. Phys., D: Appl. Phys. 50, 424002 (2017).
5. B. Nair et al., Nature (2019) doi:10.1038/s41586-019-1634-0.

Nowadays energy-intensive and expensive processes are mainly used to desalinate contaminated water and supply cooling capacity. Developing more sustainable, robust and cost-effective technologies (e.g. powered by renewable energy or waste heat) for generating clean water and cooling effect is a crucial need. This necessity is particularly important and severe in rural countries due to the lack of infrastructure and investments.
The abundant solar thermal source, which is now mainly employed to provide domestic hot water, could have a significant impact in achieving such targets, thanks to a wider engineering exploitation of innovative materials and technologies. [1,2,3] In this respect, passive distillation and cooling technologies – where all processes occur without mechanical moving parts – can make an important contribution. Unfortunately, state-of-the-art passive devices operating under one sun (that is, below 1 kW m^-2) still require efficiency enhancement, durability increase, and cost reduction to scale from lab to fab. [4,5,6] The passive technologies for solar water desalination and cooling developed at the Multi-Scale Modeling Laboratory of Politecnico di Torino (www.polito.it/small) are presented.
First, a fully passive, multistage and low-cost solar thermal device able to produce freshwater from seawater is conceived, prototyped and tested under both laboratory and outdoor conditions. The device does not require any mechanical moving parts by exploiting the capillarity of hydrophilic materials. Under realistic conditions, a distillate flow rate of almost 3 L m⁻² h⁻¹ from seawater at less than one sun is demonstrated. Theoretical models, under the same conditions, also suggested that the concept has the potential to further doubling the observed distillate rate, namely up to 6 L m^-2 h^-1. [7] Long-term operations of the passive distiller are then improved by optimizing the structure of evaporators towards enhanced salt-rejecting performance. [8] Finally, the passive distiller is optimized to recovery the low-temperature heat, to co-generate electricity and distilled water at the same time. [9]
Second, a passive device exploiting similar phenomena as i) capillarity of hydrophilic materials and the ii) salinity difference between multiple evaporators and condensers is experimentally demonstrated to provide cooling capacity up to 100 W m^-2. [10] The implemented prototype works at ambient pressure, employs environmentally friendly water solutions, and consists of several identical stages where the cooling effect comes from a cascade of evaporation-condensation processes driven by vapor pressure gradient. The experimentally validated model predicts that cooling capacities above 200 W m^-2 could be possibly achieved with a proper design optimization of the current lab-scale prototype.

Fasano, M., Humplik, T., Bevilacqua, A., Tsapatsis, M., Chiavazzo, E., Wang, E. N. and Asinari, P., Nature Communications, Vol. 7, 12762, 2016.
Ghasemi, H., Ni, G., Marconnet, A.M., Loomis, J., Yerci, S., Miljkovic, N. and Chen, G., Nature Communications, Vol 5, p.4449, 2014.
Morciano, M., Fasano, M., Salomov, U., Ventola, L., Chiavazzo, E. and Asinari, P., Scientific Reports, Vol. 7, 11970, 2017.
F. Signorato, M. Morciano, L. Bergamasco, M. Fasano, P. Asinari. submitted
Boriskina, S.V., Zandavi, S.H., Song, B., Huang, Y. and Chen, G., Optics and photonics news, Vol. 28, No. 11, 26, 2017.
Boriskina, S.V., Raza, A., Zhang, T., Wang, P., Zhou, L. and Zhu, J., MRS Bulletin, Vol. 44, No. 1, 59, 2019.
Chiavazzo, E., Morciano, M., Viglino, F., Fasano, M. and Asinari, P., Nature Sustainability, Vol. 1, No. 12, 763, 2018.
Ni, G., Zandavi, S.H., Javid, S.M., Boriskina, S.V., Cooper, T.A. and Chen, G., Energy & Environmental Science, Vol. 11, No. 6, 1510, 2018.
M. Morciano, M. Fasano, L. Bergamasco, A. Albiero, M. Lo Curzio, P. Asinari, E. Chiavazzo. submitted
M. Alberghini, M. Morciano, M. Fasano, F. Bertiglia, V. Fernicola, P. Asinari, E. Chiavazzo. submitted

In this talk, we will discuss our latest work on development of elastocaloric materials as well as cooling prototype systems. We have used additive manufacturing to engineer NiTi-based composites with extremely extremely small hysteresis in stress-strain curves. Additive manufacturing allows flexible design topology of metallic refrigerants which also act as heat exchangers. The small hysteresis results in substantially enhanced materials coefficient of performance as well as long fatigue life needed for applications. On the systems front, we have demonstrated active regeneration schemes for systems with cooling capacity up to 100s of watts using commercially available NiTi tubes. The largest temperature span we have achieved for a single stage compressive regenerators is 23 K. This work is carried out in collaboration with H. Hou, J. Cui, R. Ott, A. Stebner, D. Catalini, R. Radermacher, and Y. Hwang, N. Emaikwu, and S. Qian,

The electrocaloric (EC) effect characterizes the change in temperature induced by a change in electric field [1], with the electrocaloric coefficient being defined as α=∂T/∂Ε│_S, where T is the temperature, E is the electric field and S is the entropy. It is expected that the EC effect has the potential to be an efficient solid-state cooling devices for a broad range of applications [2,3]. Up to now, fewer investigations about EC effects have been performed in relaxor ferroelectrics [4-8]. Here we study EC effect in Ba(Zr_0.5Ti_0.5)O₃ (BZT) and PbMg_1/3Nb_2/3O₃ (PMN) relaxor ferroelectrics. We use the first-principles-based effective Hamiltonian approach combined with Monte Carlo that has been recently developed for BZT and PMN solid solutions [9,10] to investigate the EC effects.

In the lead-free BZT systems, we investigated the electrocaloric effects subject to electric fields of different magnitude and all oriented along the pseudo-cubic [001] direction. It is found that, for any temperature, α exhibits a non-monotonic behavior with field that consists of small values at low fields, followed by an increase up to a maximum before decreasing for larger fields. Below the Burns temperature, this maximum of α is demonstrated to be correlated to a very specific microscopic feature. Finally, equalities that are derived from a simple Landau model reproduce and further help to understand the anomalous behavior of α with field and temperature in BZT, for any temperature above T_m where the dielectric response can exhibit a peak. Our simulations also confirm that indirect and direct approaches yield similar results of the α EC coefficient for any temperature above the T_m temperature but differ from each other for temperature below T_m, because of the non-ergodicity adopted by BZT at these low temperatures.

We also employed the effective Hamiltonian of Ref. [10] to shed some light on electrocaloric effects in PMN. It is particularly striking that such Hamiltonian can qualitatively reproduce not only the peculiar electric field-versus-temperature phase diagram but also the optimization of the EC coefficient near the critical point in this rather complex system. The fact that the recently developed Landau-like model, predicting that the EC coefficient is simply related to the product of temperature and the field derivative of the square of the polarization [7], also describes well the EC behavior of PMN as a function of electric field and temperature is also promising for phenomenological modelization of complex inhomogeneous systems. We therefore hope that our studies [7,8] lead to a broader knowledge of EC effects in ferroelectrics.

This work is supported by the National Natural Science Foundation of China (Grants No. 11804138 and No. 11825403), Shandong Provincial Natural Science Foundation (Grant No. ZR2019QA008), China Postdoctoral Science Foundation (Grant No. 2018M641905), and Shanghai Super Postdoctoral Incentive Program. We also thank the DARPA Grant No. HR0011-15-2-0038 (MATRIX program) and ARO grant W911NF-16-1-0227.

References:
[1] J. F. Scott, Annu. Rev. Mater. Sci. 41, 229 (2011).
[2] Y. Bai, G.-P. Zheng, and S.-Q. Shi, J. Appl. Phys. 108, 104102 (2010).
[3] X. Moya, S. K.-Narayan, N. D. Mathur, Nat. Mater. 13, 439 (2014).
[4] S. G. Lu et al., Appl. Phys. Lett. 97, 202901 (2010).
[5] R. Pirc, Z. Kutnjak, R. Blinc, and Q. M. Zhang, J. Appl. Phys. 110, 074113 (2011).
[6] B. Rozic et al., J. Appl. Phys. 110, 064118 (2011).
[7] Z. Jiang, S. Prokhorenko, S. Prosandeev, Y. Nahas, D. Wang, J. Íñiguez, E. Defay, and L. Bellaiche, Phys. Rev. B 96, 014114 (2017).
[8] Z. Jiang, Y. Nahas, S. Prokhorenko, S. Prosandeev, D. Wang, J. Íñiguez, and L. Bellaiche, Phys. Rev. B 97, 104110 (2018).
[9] A. R. Akbarzadeh, S. Prosandeev, E. J. Walter, A. Al-Barakaty, and L. Bellaiche, Phys. Rev. Lett. 108, 257601 (2012).
[10] A. Al-Barakaty, S. Prosandeev, D. Wang, B. Dkhil, and L. Bellaiche, Phys. Rev. B 91, 214117 (2015).

We present barocaloric studies of ferroelectric polymers of poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE]. There copolymers are well known because of their giant electrocaloric response, but here we study pressure-driven thermal changes. PVDF-TrFE polymers undergo first-order phase transition with very large latent heats and volume changes, near room temperature. These three properties make them ideal materials for barocalorics.

Barocaloric materials are blossoming, with recent reports of thermal changes that compare to those observed in commercial refrigerant fluids [1-3]. The viability of future barocaloric chillers will rely on properties that go beyond the magnitude of the pressure-driven barocaloric effects. On the one hand, the challenge of effective transmission of hydrostatic pressure needs to be addressed in order to guarantee homogeneous stress distribution in the active material, and therefore optimal caloric performance. On the other hand, heat extraction needs to occur at rates that can match the practical cycling frequencies. Here, I will discuss key aspects of pressure transmission and heat transfer in barocaloric devices, and present a guide for barocaloric materials to match applications requirements.

[1] X. Moya, et al. "Use of barocaloric materials and barocaloric devices", patent WO2018/069506 (2018).
[2] Li, Bing, et al. "Colossal barocaloric effects in plastic crystals." Nature 567.7749 (2019): 506.
[3] P. Lloveras, et al. "Colossal barocaloric effects near room temperature in plastic crystals of neopentylglycol." Nature communications 10.1 (2019): 1803.

The massive use of vapor-compressor-based coolers poses a serious global environmental problem due to a relatively small efficiency and the use of hydrofluorocarbons with huge greenhouse potential [1]. These devices are based on controlling the exchange of latent heat involved in liquid-vapor transitions by means of hydrostatic pressure. Alternative technologies based on driving solid-state first-order phase transitions using magnetic, electric and uniaxial stress fields avoid the use of harmful fluids and promise higher efficiencies [2], but they are limited by a relatively small magnitude of the latent heat associated with such transitions compared to evaporation. Here we show that pressure-driven caloric (i.e. barocaloric) effects at phase transitions in some plastic crystals is comparable to that in hydrofluorocarbons due to the emergence of strong molecular orientational disorder [3,4]. We also discuss the reversible barocaloric response that is obtained upon cyclic application and removal of pressure as this is key for technological implementation. In particular, we characterize reversible effects in terms of magnitude, temperature spans, and minimum required pressure. We determine that, while some of the plastic crystals under analysis are unfeasible for applications due to irreversibility within a large pressure range, others demonstrate colossal barocaloric effects that are reversible at low pressures [3,4] and therefore show aptitudes that should promote the future development of environmentally-friendly solid-state barocaloric cooling devices.

[1] https://www.birmingham.ac.uk/Documents/college-eps/energy/Publications/2018-clean-cold-report.pdf
[2] X. Moya et al., Nat. Mater. 13, 439 (2014).
[3] P. Lloveras et al., Nat. Commun. 10, 1803 (2019).
[4] A. Aznar et al., submitted (2019).

Abstract body: Spin crossover occurs in compounds where the crystal field splitting of d-orbitals associated with a magnetic moment is of the order of k_BT. As a result, a change of state from low spin (LS) to high spin (HS) can occur at the so-called spin crossover temperature, T_SCO. Crucially for caloric applications, the change of state from LS to HS can be either continuous or first order, and it can occur at temperatures up to and beyond room temperature. This makes SCO transitions of interest for caloric cooling applications. Since SCO compounds are paramagnets or diamagnets, the largest caloric effects will be barocaloric rather than magnetocaloric.
In this presentation, we provide the first direct experimental demonstration of a giant, low pressure barocaloric effect (BCE) and outline future research directions for the development and optimization of the BCE associated with spin crossover.

The refrigeration and air conditioning consumes about 17% of the overall electricity used worldwide, and will drastically increase in the next 30 years. The vast majority of these systems utilize vapour-compression technology, which is actually and unfortunately still associated with the existing refrigerants and their environmental impact. Since they contribute to the 7.8 % of global greenhouse gas emissions, large efforts are invested in finding appropriate future working substances. However, the options for new, environmentally friendly refrigerants are very limited. Another important step in decreasing the energy consumption is through the energy efficient refrigeration and air conditioning. The large scale vapour compression systems operate with their upper limit of about 60% of the second law efficiency. However they suffer by the low 20% efficiency especially in small appliances. The above issues represent the driver for the quest for energy efficient and environmentally friendly refrigeration and air-conditioning, which is pronounced more than ever. Among the available best alternatives to existing technologies is the caloric energy conversion. This is due to the potentially very high energy efficiency and use of environmentally friendly and safe solid-state working substances. Most of today’s caloric devices apply so-called active regeneration, which involves the oscillation of the working fluid through the matrix of the caloric material – the caloric regenerator. However, the unavoidable, irreversible viscous and heat-transfer losses apply limits to the caloric device’s performance as well as its size. The quest for better caloric-device performance has led to the development of thermal control elements, which control the heat flux on different size and time scales. Here we describe the working principles of these elements: thermal switches, thermal diodes and thermal regulators. This is followed by the critical review of the research activities and applications of thermal control elements in all types of caloric devices. It is shown that thermal control elements have the potential to substantially improve the power density of caloric devices. Finally, we present the target features and future research guidelines for these elements in the field of caloric technologies.

Cooling, manifested both as air conditioning and as refrigeration, is regarded as one of the most energy-intensive technologies in demand today. Magnetic refrigeration is an energy-efficient, sustainable, environmentally-friendly alternative to conventional vapor-compression cooling technology. There are a number of magnetic refrigerator device designs in existence today that are predicted to be highly energy efficient, on condition that the suitable working materials can be developed.¹ This materials challenge has proven to be daunting due to issues associated with intricate synthesis/post-processing protocols and complications related to shaping the mostly brittle magnetocaloric alloys into thin-walled, channeled regenerator structures to facilitate efficient heat transfer between the solid refrigerant and the heat exchange fluid in an active magnetic regenerator (AMR) cooling device. To this end, research centered on fabrication of functionally-graded AlFe₂B₂ magnetic regenerators via sophisticated additive manufacturing (AM) methods will be presented in this talk. This ferromagnetic intermetallic boride has attracted considerable attention due to its low cost, promising thermal properties that promote effective heat transfer (specific heat capacity, C_p=115 J/moleK; thermal conductivity, κ=5.6 W/mK) and moderate room temperature magnetocaloric response (adiabatic temperature change, ΔT_ad=1 K and magnetic entropy change,ΔS=2.6 J/kgK at μ₀H_app=2 T).^2,3

In this work, powders of single phase AlFe₂B₂ were synthesized using multiple conventional routes: (i) drop-casting, followed by heat treatment at 1040 °C for 72 hours and then grinding to particulate form in air, (ii) reaction sintering at 900 °C for 24 hours, followed by etching in dilute HCl for 30 min and a (ii) molten salt-assisted solid state reaction at 600 °C for 36 hours. Experimental data indicate that irrespective of processing method, powders of dimensions ranging from 5 to 40 μm demonstrate comparable structural and magnetic properties (orthorhombic Cmmm-type crystal structure with unit cell volume, V=92.6 ± 0.4 Å; Curie temperature, T_c=277 ± 6 K) as well as similar magnetocaloric response (ΔS=2.6 ± 0.3 J/kgK at μ₀H_app=2 T). These precursor powders were used to 3D-print cylindrical AlFe₂B₂ samples using the established direct energy deposition AM method, laser enabled net shaping (LENS) and using an innovative magnetic-field assisted extrusion method whose prime novelty lies in the 3D ink formulation.^4,5 Efforts to optimize the material and printing process parameters to maximize the magnetocaloric potential of AlFe₂B₂ will be discussed. Further, the feasibility of printing high-quality magnetocaloric device components with high surface area and complex porous architectures will be deliberated upon. Overall, this study provides strategies for realizing low-cost functionally graded magnetic regenerators, thus potentially eliminating one of the main barriers to commercialization of magnetic cooling technology.

References: ¹V. Franco et. al., Annu. Rev. Mater. Res., 42, pp. 305, 2012; ²L. H. Lewis et.al., J. Alloys Compd. 650, pp. 482, 2015; ³ R. Barua et al., J. Alloys Compd., 777, pp. 1030, 2019; ⁴ B. Lejeuene et. al., Manuscript in preparation; ⁵ Zhao, Barua and Hadimani, Patent application #62.982,002.